CPSC614:Graduate Computer Architecture Network 1: Definitions, Metrics Prof. Lawrence Rauchwerger

CPSC614:Graduate Computer Architecture

Network 1: Definitions, MetricsProf. Lawrence Rauchwerger

Based on lectures of

Prof. David A. Patterson

UC Berkeley

Review: A Little Queuing Theory

• Queuing models assume state of equilibrium: input rate = output rate

• Notation: r average number of arriving customers/second

Tser average time to service a customer (tradtionally µ = 1/ Tser )u server utilization (0..1): u = r x Tser

Tq average time/customer in queue Tsys average time/customer in system: Tsys = Tq + Tser

Lq average length of queue: Lq = r x Tq

Lsys average length of system : Lsys = r x Tsys

• Little’s Law: Lengthsystem = rate x Timesystem (Mean number customers = arrival rate x mean service time)

Proc IOC Device

Queue server

System

Review: I/O Benchmarks

• Scaling to track technological change• TPC: price performance as nomalizing

configuration feature• Auditing to ensure no foul play• Throughput with restricted response

time is normal measure• Benchmarks to measure Availability,

Maintainability?

Review: Availability benchmarks

• Availability benchmarks can provide valuable insight into availability behavior of systems

– reveal undocumented availability policies– illustrate impact of specific faults on system behavior

• Methodology is best for understanding the availability behavior of a system

– extensions are needed to distill results for automated system comparison

• A good fault-injection environment is critical– need realistic, reproducible, controlled faults– system designers should consider building in hooks for fault-

injection and availability testing

• Measuring and understanding availability will be crucial in building systems that meet the needs of modern server applications

– this benchmarking methodology is just 1st step towards goal

Networks

• Goal: Communication between computers• Eventual Goal: treat collection of

computers as if one big computer, distributed resource sharing

• Theme: Different computers must agree on many things

– Overriding importance of standards and protocols– Error tolerance critical as well

• Warning: Terminology-rich environment

Networks

• Facets people talk a lot about:– direct (point-to-point) vs. indirect (multi-hop)– topology (e.g., bus, ring, DAG)– routing algorithms– switching (aka multiplexing)– wiring (e.g., choice of media, copper, coax,

fiber)

• What really matters:– latency– bandwidth– cost– reliability

Interconnections (Networks)• Examples (see Figure 7.19, page 633):

– Wide Area Network (ATM): 100-1000s nodes; ~ 5,000 kilometers– Local Area Networks (Ethernet): 10-1000 nodes; ~ 1-2 kilometers– System/Storage Area Networks (FC-AL): 10-100s nodes;

~ 0.025 to 0.1 kilometers per link

a.k.a.network,communication subnet

a.k.a.end systems,hosts

Interconnection Network

SAN: Storage vs. System

• Storage Area Network (SAN): A block I/O oriented network between application servers and storage

– Fibre Channel is an example

• Usually high bandwidth requirements, and less concerned about latency

– in 2001: 1 Gbit bandwidth and millisecond latency OK

• Commonly a dedicated network (that is, not connected to another network)

• May need to work gracefully when saturated

• Given larger block size, may have higher bit error rate (BER) requirement than LAN

SAN: Storage vs. System

• System Area Network (SAN): A network aimed at connecting computers

– Myrinet is an example

• Aimed at High Bandwidth AND Low Latency.

– in 2001: > 1 Gbit bandwidth and ~ 10 microsecond

• May offer in order delivery of packets• Given larger block size, may have

higher bit error rate (BER) requirement than LAN

More Network Background

• Connection of 2 or more networks: Internetworking

• 3 cultures for 3 classes of networks– WAN: telecommunications, Internet– LAN: PC, workstations, servers cost– SAN: Clusters, RAID boxes: latency (System

A.N.) or bandwidth (Storage A.N.)

• Try for single terminology• Motivate the interconnection

complexity incrementally

ABCs of Networks

• Starting Point: Send bits between 2 computers

• Queue (FIFO) on each end• Information sent called a “message”• Can send both ways (“Full Duplex”)• Rules for communication? “protocol”

– Inside a computer: » Loads/Stores: Request (Address) & Response

(Data)» Need Request & Response signaling

A Simple Example

• What is the format of mesage?– Fixed? Number bytes?

Request/Response

Address/Data

1 bit 32 bits0: Please send data from Address1: Packet contains data corresponding to request

• Header/Trailer: information to deliver a message

• Payload: data in message (1 word above)

Questions About Simple Example

• What if more than 2 computers want to communicate?

– Need computer “address field” (destination) in packet

• What if packet is garbled in transit?– Add “error detection field” in packet (e.g., Cyclic Redundancy Chk)

• What if packet is lost?– More “elaborate protocols” to detect loss

(e.g., NAK, ARQ, time outs)

• What if multiple processes/machine?– Queue per process to provide protection

• Simple questions such as these lead to more complex protocols and packet formats => complexity

A Simple Example Revisted

• What is the format of packet?– Fixed? Number bytes?

Request/Response

Address/Data

1 bit 32 bits

00: Request—Please send data from Address01: Reply—Packet contains data corresponding to request10: Acknowledge request11: Acknowledge reply

4 bits

CRC

Software to Send and Receive

• SW Send steps1: Application copies data to OS buffer

2: OS calculates checksum, starts timer

3: OS sends data to network interface HW and says start

• SW Receive steps3: OS copies data from network interface HW to OS buffer

2: OS calculates checksum, if matches send ACK; if not, deletes message (sender resends when timer expires)

1: If OK, OS copies data to user address space and signals application to continue

• Sequence of steps for SW: protocol– Example similar to UDP/IP protocol in UNIX

Network Performance Measures

• Overhead: latency of interface vs. Latency: network

Universal Performance Metrics

Sender

Receiver

SenderOverhead

Transmission time(size ÷ bandwidth)

Transmission time(size ÷ bandwidth)

Time ofFlight

ReceiverOverhead

Transport Latency

Total Latency = Sender Overhead + Time of Flight + Message Size ÷ BW + Receiver Overhead

Total Latency

(processorbusy)

(processorbusy)

Includes header/trailer in BW calculation?

Total Latency Example

• 1000 Mbit/sec., sending overhead of 80 µsec & receiving overhead of 100 µsec.

• a 10000 byte message (including the header), allows 10000 bytes in a single message

• 3 situations: distance 1000 km v. 0.5 km v. 0.01• Speed of light ~ 300,000 km/sec (1/2 in media)

• Latency0.01km =

• Latency0.01km =

• Latency1000km =

Total Latency Example

• 1000 Mbit/sec., sending overhead of 80 µsec & receiving overhead of 100 µsec.

• a 10000 byte message (including the header), allows 10000 bytes in a single message

• 2 situations: distance 100 m vs. 1000 km• Speed of light ~ 300,000 km/sec

• Latency0.01km = 80 + 0.01km / (50% x 300,000) + 10000 x 8 / 1000 + 100 = 260 µsec

• Latency0.5km = 80 + 0.5km / (50% x 300,000) + 10000 x 8 / 1000 + 100 = 263 µsec

• Latency1000km = 80 + 1000 km / (50% x 300,000) + 10000 x 8 / 1000 + 100 = 6931

• Long time of flight => complex WAN protocol

Universal Metrics

• Apply recursively to all levels of system

• inside a chip, between chips on a board, between computers in a cluster, …

• Look at WAN v. LAN v. SAN

Simplified Latency Model

• Total Latency = Overhead + Message Size / BW

• Overhead = Sender Overhead + Time of Flight + Receiver Overhead

• Example: show what happens as vary – Overhead: 1, 25, 500 µsec– BW: 10,100, 1000 Mbit/sec (factors of 10)– Message Size: 16 Bytes to 4 MB (factors of 4)

• If overhead 500 µsec, how big a message > 10 Mb/s?

Overhead, BW, Size

0

0

1

10

100

1,000

16

64

25

6

10

24

40

96

16

38

4

65

53

6

26

21

44

10

48

57

6

41

94

30

4

Message Size (bytes)

Eff

ect

ive

Ba

nd

wid

th (

Mb

it/s

ec)

o1,bw1000

o1,bw100

o1,bw10 o500,

bw10

o500,bw100

o500,bw1000

o25,bw100

o25,bw1000

o25,bw10

Msg Size

Delivered BW

•How big are real messages?

Measurement: Sizes of Message for NFS

• 95% Msgs, 30% bytes for packets ~ 200 bytes• > 50% data transfered in packets = 8KB

Packet size

Cu

mm

ula

tive %

0%10%20%30%40%50%60%70%80%90%

100%

0 1024 2048 3072 4096 5120 6144 7168 8192

Msgs

Bytes Why?

Impact of Overhead on Delivered BW

• BW model: Time = overhead + msg size/peak BW

Peak BW (MB/sec)

Delivere

d B

W(M

B/s

ec)

0.10

1.00

10.00

100.00

1000.00

1

10

10

0

10

00

1

10

100

1000

MinTimeone-way

µsecs

Interconnect Issues

• Performance Measures• Network Media

Network Media

Copper, 1mm think, twisted to avoidattenna effect (telephone)"Cat 5" is 4 twisted pairs in bundle

Used by cable companies: high BW, good noise immunity

Light: 3 parts are cable, light source, light detector.Note fiber is unidirectional; need 2 for full duplex

Twisted Pair:

Coaxial Cable:

Copper coreInsulator

Braided outer conductor

Plastic Covering

Fiber Optics

Transmitter– L.E.D– Laser Diode

Receiver– Photodiode

lightsource Silica core

Total internalreflection

Cladding

Cladding

Buffer

Buffer

Fiber• Multimode fiber: ~ 62.5 micron diameter vs. the 1.3

micron wavelength of infrared light. Since wider it has more dispersion problems, limiting its length at 1000 Mbits/s for 0.1 km, and 1-3 km at 100 Mbits/s. Uses LED as light

• Single mode fiber: "single wavelength" fiber (8-9 microns) uses laser diodes, 1-5 Gbits/s for 100s kms– Less reliable and more expensive, and restrictions on bending

– Cost, bandwidth, and distance of single-mode fiber affected by power of the light source, the sensitivity of the light detector, and the attenuation rate (loss of optical signal strength as light passes through the fiber) per kilometer of the fiber cable.

– Typically glass fiber, since has better characteristics than the less expensive plastic fiber

Wave Division Multiplexing Fiber

• Send N independent streams on single fiber!

• Just use different wavelengths to send and demultiplex at receiver

• WDM in 2000: 40 Gbit/s using 8 wavelengths

• Plan to go to 80 wavelengths => 400 Gbit/s!

• A figure of merit: BW* max distance (Gbit-km/sec)

• 10X/4 years, or 1.8X per year

Compare Media• Assume 40 2.5" disks, each 25 GB, Move 1 km• Compare Cat 5 (100 Mbit/s), Multimode fiber (1000

Mbit/s), single mode (2500 Mbit/s), and car• Cat 5: 1000 x 1024 x 8 Mb / 100 Mb/s = 23 hrs• MM: 1000 x 1024 x 8 Mb / 1000 Mb/s = 2.3 hrs• SM: 1000 x 1024 x 8 Mb / 2500 Mb/s = 0.9 hrs• Car: 5 min + 1 km / 50 kph + 10 min = 0.25 hrs• Car of disks = high BW media

Interconnect Issues

• Performance Measures• Network Media• Connecting Multiple Computers

Connecting Multiple Computers

• Shared Media vs. Switched: pairs communicate at same time: “point-to-point” connections

• Aggregate BW in switched network is many times shared

– point-to-point faster since no arbitration, simpler interface

• Arbitration in Shared network?– Central arbiter for LAN?– Listen to check if being used (“Carrier

Sensing”)– Listen to check if collision

(“Collision Detection”)– Random resend to avoid repeated collisions;

not fair arbitration; – OK if low utilization (A. K. A. data switching

interchanges, multistageinterconnection networks,interface message processors)

Summary: Interconnections

• Communication between computers• Packets for standards, protocols to cover

normal and abnormal events• Performance issues: HW & SW overhead,

interconnect latency, bisection BW• Media sets cost, distance• Shared vs. Swicthed Media determines BW

Projects

• See www.cs/~pattrsn/252S01/suggestions.html

If time permits

• Discuss Hennessy paper. "The future of systems research." Computer, vol.32, (no.8), IEEE Comput. Soc, Aug. 1999

• Microprocessor Performance via ILP Analogy?

• What is key metric if services via servers is killer app?

• What is new focus for PostPC Era?• How does he define availability vs.

textbook?